Sensor elements for sensing at least a proportion of at least one gas component of a gas in a measurement gas space are described in, for example, Robert Bosch GmbH: “Sensoren im Kraftfahrzeug, 1. Auflage 2010” [“Sensors in motor vehicles, 1st edition 2010”], pages 160-165. The sensor devices may, in particular, be lambda probes. Lambda probes based on an ionically conductive sensor element, typically made of zirconium dioxide (ZrO2), generally compare an electrochemical potential of an electrode on the measurement gas side with an electrochemical potential of a reference electrode. The reference electrode is preferably situated in a defined reference gas, typically air. A measured variable collected for the sensing operation may usually be, for example, a Nernst voltage which develops between the two electrodes, the Nernst voltage generally being the greater, the greater is, in particular, a partial pressure difference, for example of oxygen (O2), between the two electrodes.
German Patent Application No. DE 100 43 089 A1 describes, for example, a sensor for determining gas components and/or concentrations of gas constituents in gas mixtures, the sensor having a reference electrode which may be exposed, via a reference gas channel, to a reference gas, in particular air or an oxygen-containing gas. In one advantageous embodiment, there is provided beside the reference gas channel from the internal volume of the sensor structure to the reference electrode a further gas channel, in particular having a catalytically active region or a catalytically active surface. The further gas channel may, for example, be configured as a double reference channel that is capable of taking on, for example, the reaction of hydrocarbons (HC).
Signal accuracy may be increased, for example, by an open reference channel, in particular of a small cross-section, as described, for example, in German Patent Application No. DE 10 2010 039 392.
German Patent No. DE 100 51 833 A1 describes a planar gas sensor element for the determination of gas components, wherein, in particular, electrodes at different temperature levels and on the reference side may be shifted. The reference electrode is in the form of a plurality of partial surfaces connected to one another in an electrically conductive manner. Owing to the division into a plurality of partial surfaces, the reference electrode, which, in particular, is of a large surface area, is not heated uniformly throughout, so that there are always hot spots and cold spots over the surface of the reference electrode, which distinctly reduces the effect of an exhaust gas temperature, the exhaust gas flow and the heating power of a heating element on a Nernst voltage produced. According to that patent, it is said to be advantageous if division of the reference electrode into a plurality of partial surfaces connected to one another in an electrically conductive manner is performed in such a way that those partial surfaces of the reference electrode that are at least to a large extent in a hot region have at least approximately the same internal resistance as those partial surfaces that are at least to a large extent outside the hot region.
In the case of conventional sensor devices, especially in the case of lambda probes, non-combusted hydrocarbons may be present, in particular in the reference gas channel, which usually consume a proportion of the oxygen supplied to the reference electrode, with the result that the oxygen concentration at the reference electrode may be reduced, thereby interfering with probe function. That behavior is also known as CSD (characteristic shift down, also referred to as continuous shift down or chemical shift down). Furthermore, non-combusted hydrocarbons may preferably be oxidized over hot, catalytically active surfaces, especially at the reference electrode.
To avoid contamination of the reference air by rich gases, in particular CSD, generally two concepts are pursued in the related art:
On the one hand, in the case of a reference air channel, an artificial reference may be produced by a reference pump current, for example as described in German Patent Nos. DE 199 63 566 A1 and DE 43 33 230 A1. Preferably, the reference electrode may be exposed to the artificial reference, which may be produced, for example, by a reference pump current from an outer electrode to the reference electrode. The reference pump current may be obtained, for example, by suitable connection of the sensor device, especially a probe, in a control device. For this mode of operation, the probe design should have as a rule an impermeable reference channel, for example a reference air channel with a high diffusion resistance, for example one that is narrow and/or has a low porosity, for example in order that oxygen, which, in particular, may be pumped to the reference electrode especially by the reference pump current, stays at the reference electrode and/or in order that as little rich gas as possible is able to reach the reference electrode, while a reference gas channel should preferably be formed with a more large-pored material so that bursting of the sensor element, for example due to an oxygen overpressure at the reference electrode, is preferably prevented. That form of implementation is generally also referred to as a “pumped reference”.
In a further form of implementation, an open and/or very porous reference air channel is generally used, such a form of implementation generally being termed an “air reference”, and the reference electrode is preferably exposed directly to ambient air. This makes it possible, for example, for the hydrocarbons to be reacted to exhaustion by oxygen that is able to reach the reference electrode through the open and/or very porous reference air channel. Preferably, no reference pump current is impressed from the outer electrode to the reference electrode. The reference air channel preferably has a low diffusion resistance, for example by virtue of being a wide reference air channel, preferably with a high porosity.
A particular disadvantage is that, depending on the mode of operation, in particular pumped reference or air reference, two different designs and/or types of probe are usually necessary.
It would therefore be desirable to have a sensor device, in particular a lambda probe, for example a λ=1−probe, capable of being operated selectively with a pumped reference or with an open reference, in particular with an air reference. In that way it is also possible, for example, for the number of control devices that are compatible with the sensor device, in particular the probe, to rise significantly.